The present invention relates to an electric dust precipitator and, more particularly, to an improvement of an electric dust precipitator comprising a hammering assembly that applies mechanical impact to collecting electrodes or precipitation electrodes to shake off the collected dust on the surface thereof, as well as an improvement of the collecting electrodes therein.
Referring to FIGS. 1 and 2, a conventional hammering assembly for an electric dust precipitator is described for its configuration and electrical connections with collecting electrodes and discharge electrodes. In a charging division housing in the electric dust precipitator, grounded collecting electrodes 11 through 15 (15 is omitted in FIG. 1) are placed. Each of the collecting electrodes 11 through 15 is provided with a hammering assembly comprising a hammer 20 and a struck seat 24 which is provided at a lower portion of the corresponding collecting electrode. The hammers 20 are pivotally mounted on a common shaft 21 with fork ends 22 and pins 23. Rotation force is transmitted from a motor (not shown) to the shaft 21 via a reduction gear mechanism, which causes the shaft 21 to rotate at a low speed in the direction depicted by an arrow in the figure. Rotation of the shaft 21 lifts the hammer 20 to the uppermost point of its movement. The hammer 20 then swings because of the gravity and strikes the struck seat 24. The vibration generated by the hammer strike is transmitted to the collecting electrode and the collected dust is shaken off from the collective electrode surface.
Since the hammers 20 are attached to the shaft 21 at a different angle with respect to each other, the hammer 20 strikes the corresponding struck seat 24 from one to another. This is for preventing the collected dust from re-scattering simultaneously at two or more collecting electrodes after having been shaken off by the hammers. In other words, the collected dust is re-scattered in turn to make re-scattered puff (a cloud of visible smoke) inconspicuous that is escaped through a chimney into the air.
On the other hand, a plurality of discharge electrodes are placed between the adjacent collecting electrodes. In FIG. 2, four discharge electrodes A1 through A4 are placed between the collecting electrodes 11 and 12. Likewise, four discharge electrodes B1 through B4 are placed between the collecting electrodes 12 and 13, four discharge electrodes C1 through C4 are placed between the collecting electrodes 13 and 14, and four discharge electrodes D1 through D4 are placed between the collecting electrodes 14 and 15. All discharge electrodes are connected to a common power supply unit 30. The power supply unit 30 comprises a transformer 31, a rectifier bridge circuit 32 connected to a secondary output of the transformer 31, and a spark detection voltage divider 33. A primary input of the transformer 31 is connected to an alternative current power supply AC through two controlling thyristors 35. A control unit 36 controls a conduction angle of the thyristor 35, which in turn controls the voltage across the secondary output of the transformer 31. More specifically, the control unit 36 monitors the voltage across the spark detection voltage divider 33 to determine the presence or absence of spark between the discharge electrode and the collecting electrode and controls the voltage across (or the current flowing through) the secondary output to have a proper value.
Though only five collecting electrodes and sixteen discharge electrodes are illustrated in FIGS. 1 and 2, an actual electric dust precipitator has several tens of collecting electrodes aligned transversally with respect to a gas flow and several hundreds of discharge electrodes.
As described above in conjunction with FIG. 1, the collected dust is shaken off from the surface of the collecting electrodes upon their vibration generated by the hammers. Although the dust is then fallen down as a mass into a lower hopper, a part of the falling dust is carried by the gas flow. This phenomenon is referred to as xe2x80x9cre-scatteringxe2x80x9d by hammering and has been an obstacle to improved dust collecting capacity of the electric dust precipitator. In addition, hammering may not shake the collected dust completely off the surface of the collecting electrodes. The collected dust remained on the surface may re-scatter without being hammered off. Furthermore, accumulation of the collected dust on the electrode surface, if happened, may be a cause of back discharge and spark. Such a hammering technique is thus desired that can shake off the collected dust as much as possible without causing re-scattering thereof.
Conventional hammering has been made while applying the voltage to the discharge electrodes of the electric dust precipitator. For more complete shaking off of the collected dust, the hammering may be made by means of hammering with reduced voltage than usual or a power off rapping that the hammer strikes the collecting electrode with the power supply completely turned off. Since the hammering with the reduced voltage and the power off rapping require to reduce or eliminate the voltage, it may deteriorates the dust collecting function of the electric dust precipitator or even prevents the precipitator from displaying its function.
In the hammering assembly, slow rotation of the shaft 21 lifts the hammers 20 one by one to their respective uppermost point and the hammers 20 swing due to the gravity from the upper limit of their movement to the struck seat 24. It takes several minutes for the shaft 21 to rotate once, so that the hammering with the reduced voltage or the power off rapping continues for at least several minutes and that the dust collecting capacity may be deteriorated during this length of time. The power off rapping is seldom used due to this problem. The power off rapping is used only under limited conditions. The limited conditions are the case that the dust collecting capacity has sufficient margins during operation of a plant including the electric dust precipitator at a significantly lower load than in the normal operating status. In other words, the power off rapping may be used if necessary only when sufficient dust collecting capacity can certainly be achieved even after down-time of one of the charging division housings in the electric dust precipitator.
The present inventor has found that the hammering based on the conventional hammering technique with the application of the voltage can shake off the dust only incompletely with a disadvantageously and relatively large amount of re-scattering dust. The present inventor has developed an apparatus to monitor motion of dust particles in the air separated by the hammering from the collecting electrodes of the electric dust precipitator and has examined behavior of the separated dust particles.
This is described more in detail below with reference to FIG. 3. According to the results of the observation, the collected dust can hardly be shaken off by the hammering without being accumulated to a significant thickness on the collecting electrodes 11 and 12 at a position opposed to a corona generating region Cg (R1 region in FIG. 3). On the other hand, the collected dust is shaken off from the surface of the collecting electrodes 11 and 12 at a remaining position away from the R1 region and not being opposed to the corona generating region Cg. Instead, the collected dust scatters in the air (R2 region in FIG. 3). In this way, it has been found that the shaking off is incomplete at and around the R1 region while the re-scatter by the hammering occurs at and around the R2 region.
A reason for this difference between the R1 and the R2 regions would be as follows. A large amount of negative ions generated around the discharge electrode A collides against the surface of the collected dust at the R1 region, producing negative charges on the dust. The collecting electrodes 11 and 12 attract the charged dust strongly and the latter is difficult to be shaken off.
On the contrary, few or no negative ions collide against the surface of the collected dust at the R2 region. In this case, electric field is generated by the voltage between the discharge electrode A and the collecting electrodes 11 and 12, the electric field produces positive charges in the direction from the collecting electrodes (metallic portion) to the surface of the collected dust, and the positive charges are accumulated onto the surface of the collected dust as the positive charges. The positive charges on the surface of the accumulated or the collected dust are not neutralized because few or no negative ions collides from outside against the dust. Accordingly, the collected dust is attracted by the discharge electrode A and re-scatters in the air after being shaken off by the hammering.
The re-scatter of the dust occurs in a larger area at the R2 region during the hammering with the reduced voltage because there is less negative ions than in the power off rapping. The voltage resides for a significant length of time after discontinuing application of the voltage because of a capacitance between the discharge electrode and the collecting electrodes in the power off rapping. The re-scattering of the dust by the sequential hammering occurs on the entire surface of the collecting electrodes 11 and 12 until the voltage is eliminated.
The above-mentioned problems have been an obstacle to a hammering technique that allows more complete shaking off of the dust while scattering less or no dust in the air.
In spite of the above circumstances, recent electric dust precipitators are required to achieve an outlet dust concentration of 10 mg/m3 N or less. Therefore, reduction of the amount of the dust re-scattered by the hammering is essential from the environmental viewpoints.
With respect to the above-mentioned problems, an object of the present invention is to provide an electric dust precipitator that can provide a high dust collecting capacity by means of reducing an amount of the dust re-scattered by hammering.
Another object of the present invention is to provide an electric dust precipitator of which dust collecting capacity is less or not deteriorated even during power off rapping.
It is yet another object of the present invention to provide a collecting electrode suitable for the above-mentioned electric dust precipitator.
An electric dust precipitator according to a first invention comprises a plurality of collecting electrodes and a plurality of hammering assemblies for collected dust, each of which is mounted on each of the collecting electrodes and each of which has a hammer.
According to an aspect of the present invention, a plurality of discharge electrodes are grouped into a plurality of groups and the discharge electrodes of each group are placed in each of gas passages each of which is defined by the plurality of collecting electrodes. The discharge electrodes in each group are connected to a common power supply unit for receiving output voltage through a diode provided for each group and are connected to a spark electrode. The spark electrode is positioned such that it sparks between the hammer in a passageway of the hammer upon striking. Each group of the discharge electrodes has a ground unit to be grounded by the spark just before the collecting electrodes on both sides thereof are struck.
An electric dust precipitator according to a second invention comprises a plurality of collecting electrodes housed in each charging division housing and a plurality of hammering assemblies for collected dust, each of which is mounted on each of the collecting electrodes and each of which has a hammer.
According to another aspect of the present invention, it is constructed so that the collecting electrodes are struck by the hammers at the same time. All discharge electrodes in the charging division housing are connected to a common power supply unit to receive output voltage therefrom and are connected to a common spark electrode. The spark electrode is positioned such that the spark electrode sparks, just before the striking by the hammer, between it and the hammer or another moving member that moves in cooperation with or in synchronism with the hammer. Each group of the discharge electrodes has a ground unit to be grounded by the spark just before the collecting electrodes are struck.
The collecting electrode according to the present invention is applied to an electric dust precipitator comprising a plurality of collecting electrodes housed in each charging division housing and a plurality of hammering assemblies for collected dust, each of which is mounted on each of the collecting electrodes and each of which has a hammer. Each collecting electrode is formed of two plates placed in parallel with a gap of several millimeters or larger to form a dust chute therebetween. The plates are louver-shaped. The louver is designed to receive the dust separated and fallen from the outside the plates and guide it into the dust chute.